J. Phys. Chem. 1989, 93, 509-511
509
Photochemistry of Solid Ozone Arthur J. Sedlacek and Charles A. Wight* Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 (Received: August 23, 1988)
Samples of neat solid ozone and ozone trapped in excess ice have been subjected to laser photolysis at 308 nm. Cross sections for photoabsorption and photodestruction of the ozone are reported. The quantum yield decreases from 1.5 f 0.2 in pure ozone to 0.4 f 0.2 for ozone in excess ice. These yields are consistent with a reaction mechanism in which electronically excited O(lD) atoms are responsible for the photochemistry. In neat ozone, the atoms react with a neighboring ozone molecule to form two oxygen molecules. In water, O('D) reacts to form hydrogen peroxide, HOOH. Ground-state oxygen atoms produced in the initial photolysis of ozone most likely undergo recombination with O2 to regenerate 0,.
Introduction Depletion of atmospheric ozone is a major environmental problem, especially in the polar regions.' Several studies have examined the photodissociation of ozone and subsequent reaction of the fragments with 03,2-'' H20,12-23and other small molec u l e ~ . ~However, ~~' the importance of ozone photochemistry in
(1) Molina, M. J.; Tso, T.-L.; Molina, L. T.; Wang, F. C.-Y. Science 1987, 238, 1253. For papers on the subject, also see: Geophys. Res. Lett. 1986,13, 45. (2) Valentini, J. J. J . Chem. Phys. 1987, 86, 6759. (3) Wine, P. H.; Nimvich, J. M.; Thompson, R. J.; Ravishankara, A. R. J. Phys. Chem. 1983, 87, 3948, and references therein. (4) C o b , C.; Castellano, E.; Schumacher, H. J. J. Photochem. 1983,21, 291. (5) Arnold, I.; Comes, F. J. Chem. Phys. 1980,47, 125. Arnold, I.; Comes, F. J. J. Mol. Srrucr. 1980, 61, 223. (6) Findlay, F. D.; Snelling, D. R. J . Chem. Phys. 1971, 54, 2750. (7) Jones, I. T. N.; Wayne, R. P. Proc. R. SOC.London 1970, A319,273. ( 8 ) Clark, I. D.; Jones, I. T. N.; Wayne, R. P. Proc. R . Soc. London 1970, A31 7,407. (9) Norrish, R. G. W., F. R. S.; Wayne, R. P. Proc. R . Soc. London 1965, A288, 200. (10) Benson, S. D.; Axworthy, A. E., Jr. J . Chem. Phys. 1957,26, 1718. (1 1) McGrath, W. D.; Norrish, R. G. W., F. R. S. Proc. R . SOC.London 1959, A254, 317. (12) Lissi, E.; Heicklen, J. J . Phorochem. 1972, I , 39. (13) Burrows, J. P.; Cox, R. A.; Denvent, R. G. J. Photochem. 1981,16, 147. (14) Butler, J. E.; Talley, L. D.; Smith, G. K.;Lin, M. C. J . Chem. Phys. 1981, 74, 4501. (15) DeMore, W. B.; Tschvikow-Roux, E. J. Phys. Chem. 1974,78,1447. (16) Langley, K. F.; McGrath, W. D. Planet. Space Sci. 1971, 19, 413. (17) DeMore, W. D. J . Chem. Phys. 1967,46, 2777. (18) Norrish, R. G. W., F. R. S.; Wayne, R. P. Proc. R . Soc. London 1965, A288, 361. (19) Taube, H. Trans. Faraday Soc. 1957, 53, 656. (20) Kilpatrick, M. L.; Herrick, C. C.; Kilpatrick, M. J . Am. Chem. Soc. 1956, 78, 1784. (21) Alder, M. G.; Hill, G. R. J. Am. Chem. Soc. 1950, 72, 1884. (22) Forbes, G. S.; Heidt, L. J. J . Am. Chem. Soc. 1934, 56, 1671. (23) Mdjrath, W. D.; Norrish, R. G. W., F. R. S. Nature 1958,182,235. (24) Aminoto, S. T.; Force, A. P.; Gulotty, R. G., Jr.; Wiesenfeld, J. R. J. Chem. Phys. 1979, 71, 3640. (25) Wiesenfeld, J. R. Acc. Chem. Res. 1982, 15, 110. (26) Whitten, R. C.; Prasad, S. S. Ozone in the Free Atmosphere; Van Nostrand Reinhold: New York, 1985. (27) Schiff, H. 1. Ann. Geophys. 1972, 77, 67. (28) Aker, P. M.; Sloan, J. J. J. Chem. Phys. 1986, 85, 1412. (29) Butler, J. E.; Jurisch, G. M.; Watson, I. A.; Wiesenfeld, J. R. J . Chem. Phys. 1986,84, 5365. (30) Aker, P. M.; O'Brien, J. J. A.; Sloan, J. J. J . Chem. Phys. 1985,84, 745.
0022-3654/89/2093-0509$01.50/0
stratospheric ice crystals is largely ~ n k n o w n and , ~ ~to~ our ~~ knowledge no experimental investigation of this reaction in the solid state has been reported until now. The work of Vaida and c o - w ~ r k e r on s ~the ~ ~effects ~ of intermolecular interactions on the electronic structure of simple molecules suggests that the photochemistry could be very different from that observed in the gas phase. This type of perturbative effect on the electronic spectrum of ozone is the subject of the accompanying paperU3' Here, we report cross sections for photodestruction of solid ozone (neat and in excess ice) at 308 nm, as well as measurements of the photoabsorption cross sections. The ratio of these is the photodestruction quantum yield, which decreases from 1.5 f 0.2 in pure ozone to 0.4 f 0.2 for ozone in excess ice. These results are discussed in terms of the reaction dynamics in an environment that severely restricts molecular motion.
Experimental Section Details of the experimental apparatus and techniques are presented e l ~ e w h e r e . ~Briefly, ~ . ~ ~ mixtures of gaseous reagents are deposited directly onto an optical window (quartz, CaF2, or CsI) which is cooled to 10-77 K with a closed-cycle helium refrigerator. All of the results are temperature independent in this range. Ozone was prepared by electric discharge of oxygen, followed by distillation. Deposition rates were 3 pmol/min, and the total amount of ozone deposited was typically 30 pmol. The photodestruction cross section was determined by measuring the UV absorbance (230-310 nm, Varian DMS-90 spectrometer) before and after irradiation with the 308-nm output of a XeCl excimer laser (Questek Model 2200). The laser fluence was 2.5 mJ/cm2/pulse as measured with an absorbing disk calorimeter (Scientech Model 38-01). In some experiments, infrared absorption spectra of the samples were obtained with a Digilab Model FTS-40infrared spectrometer.
Results and Discussion The photodestruction cross section was determined by plotting In (AIA,) vs total laser fluence (photons/cm2), as shown in Figure 1 (inset). For optically thin samples, the slope of this linear plot is equal to -q.Absorbance at 308 nm was typically 0.1. Analysis (31) Butler, J. E.; McDonald, R. G.; Donaldson, D. J.; Sloan, J. J. Chem. Phys. Lett. 1983, 95, 183. (32) Crutzen, P. J.; Arnold, F. Nature 1986, 324, 651. (33) McElrov. M. G.: Salawitch. R. J.: Wofsv. S. C. Geoohvs. Res. Lett. 1986, 13, 1296.-Toon,0. B.; Hamill, P.; Turco; R. P.; Pink, j.Ibid. 1986, 13, 1284. Hamill, P.; Toon, 0. B.; Turco,R. P. Ibid. 1986, 13, 1288. (34) Donaldson, D. J.; Vaida, V.; Naaman, R. J . Phys. Chem. 1988.92, 1204. Donaldson, D. J.; Vaida, V.; Naaman, R. J. Chem. Phys. 1987, 87, 2522. (35) Sapers, S . P.; Vaida, V.; Naaman, R. J. Chem. Phys. 1988,88,3638. (36) Donaldson, D. J.; Gaines, G. A.; Vaida, V. J . Phys. Chem. 1988, 92, 2766. (37) Vaida, V.; Donaldson, D. J.; Strickler, S.; Stephens, S.; Burks, J. W. J . Phys. Chem., preceding paper in this issue. (38) Sedlacek, A. J.; Manseuto, E. S.;Wight, C. A. J. Am. Chem. SOC. 1987, 109, 6223. (39) Manseuto, E. S.; Ju, C Y . ; Wight, C. A. J. Phys. Chem., in press.
0 1989 American Chemical Society
510
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The Journal of Physical Chemistry, Vol. 93, No. 2, 1989
I
Letters TABLE I: Relevant State-Specific RCXC~~OM of Ozone and WateP reaction AH, kcal/mol O('P) + O,('A) 202(3Z;) -92.2 SA O('D) O,('A) 2OJZ -) -137.4 SA o,(~z;) + o,(~A) 202(9z;) o(3~) 25.0 SA 02('AJ + O,(IA) 202('Z -) O('P) 2.9 SA 02(1zg+) O,('A) 202(3E -1 + 0 0 ~ ) -12.0 SA O('P) H,O('A) HOOHPA) -32.9 SF O('D) H2O('A) HOOH('A) -77.7 SA O('P) H,O('A) 20H(211) 16.8 SA O('D) + H20('A) 20H(211) -29.2 SA
-----
+
L 0
0
w
i 4 0 t
200
4
NO. LASER PULSES
i
+ + +
+
+ +
"SF = spin forbidden and SA = spin allowed. TABLE II: Primary Photolysis Channels for Ozone in Visible and
Q
200
220
240
280
260
300
320
340
WAVELENGTH (nrn)
Figure 1. Absorption cross section (base e) as a function of wavelength
for neat solid ozone at 10 K. The inset is a plot of ozone absorbance (at 270 nm) vs the number of laser pulses (fluence = 4.0 X 10I5photons/ cm*/pulse). The photodestruction cross section is obtained from the slope of this plot (see text for details). of photochemical measurements in strongly absorbing media is the subject of a recent paper.40 Light scattering by some samples made it difficult to accurately determine the absorbance base line. These cross sections were evaluated by using a modified Guggenheim method4' "4
= %$+A4 exp('Jd&)
+
- exp(gd&'))
(1)
where a. and are the absorbances at fluences of 4 and 4 A& respectively, and a, is the absorbance base line. Both types of data analysis yielded consistent values of Ud. It should be noted that while light scattering sometimes influenced the absorbance measurements a t short wavelengths (230-270 nm) it did not significantly affect the amount of light absorbed at the photolysis wavelength (308 nm). This was confirmed by direct measurements of the incident and transmitted laser beam intensities. For pure cm2 (four experiments). ozone samples a d = (2.43 f 0.05) x For ozone in excess ice (H20:03= 10-50) (Td = (1.8 f 0.4) X cm2 (five experiments). All uncertainties are quoted as 95% confidence limits. Cross sections are base e. In each experiment, the photoabsorption cross section at 308 nm was determined by measuring the absorbance at that wavelength. For experiments in which light scattering was significant, the absorbance base line, a,, was determined from the intercept of the Guggenheim plot. In the calculation of 6, from the measured absorbance, we assume that 11% of the total sample is deposited in 1 cm2 at the center of the cold window. This value is based on the geometry of the deposition tube and window3*and has been confirmed by other experiment^.'^ Nevertheless, it represents a major source of uncertainty in the determination of 6,. For pure solid ozone at 308 nm ua = (1.7 f 0.2) X cm2. This is essentially the same as the gas-phase value reported by Davenport,"2 (1.51 f 0.21) X cm2. For ozone in excess ice, partial decomposition of the ozone occurred in the vacuum manifold during deposition. The actual amount of sample deposited was determined by measuring the integrated IR band intensities at 1090 and 2104 cm-l. These were compared with a sample of pure ozone deposited under conditions where decomposition was negligible. The IR bands of ozone are not strongly affected by the presence of H 2 0 , as shown by the invariance of the relative intensities of the two IR bands under all experimental conditions we have investigated thus far. The 308-nm photoabsorption cross section for ozone in excess ice was
+
(40) Sheats, J. R.; Diamond, J. J.; Smith, J. M. J . Phys. Chem. 1988, 92, 4922. (41) Guggenheim, E. A. Philos. Mag. 1926, 2, 538. Also see: Moore, J. W.; Pearson, R. G. Kinetics and Mechanism, 3rd 4.; Wiley: New York,
1981.
uv
--
Dhotolvsis channel o,(~A)
---
wavelennth threshold. nm I
02()z,-) + o(3~)
1180 61 1 463 41 1
+ +
02(IAg) O('P) o2(l2.+)OPP) 0 2 .
310
02(lA;j+ O('Dj 02('Zg+) + O('D)
266
determined to be ua = (4.0 f 1.9) X cm2. The ratio of the photodestruction and photoabsorption cross sections is the photochemical quantum yield (number of ozone molecules destroyed per photon absorbed by the sample). In pure ozone the quantum yield is 1.5 f 0.2 whereas in excess ice it decreases to 0.4 f 0.2. In the gas phase,12 the photochemical quantum yield of pure ozone is nearly 6. In the presence of excess water vapor it increases to a limiting value of 17. In the following discussion, we speculate on some of the reasons for the smaller quantum yields observed in the solid as compared with the gas phase. Primary photolysis of gas phase ozone a t 308 nm is believed to occur by two spin-allowed channels, reactions 2 and '3: where
4 is the quantum yield for O(lD) formation. The oxygen atom may react with another molecule of ozone to form two oxygen molecules. In the solid state, we postulate that only O(lD) atoms react with neighboring ozone molecules. The most likely fate of the O(3P) atoms is recombination with O2 to form ozone.
--
+ O3 0 ( 3 ~ +) o2
O('D)
202
(4)
O3
(5)
We further postulate that electronically excited O2molecules do not contribute to the destruction of ozone in the solid state. Table I shows that the reaction of O2('Zg+)is exothermic, but this species can only be produced at 308 nm via a spin-forbidden process forming O(3P), as shown in Table 11. McGrath and NorrishU have reported evidence that the reaction of O('D) with O3 produces a vibrationally excited O2molecule with sufficient energy to further react with another O3 and regenerate O(lD). However, vibrational deactivation of O2in the solid state should rapidly lower its energy to below the reaction threshold ( u = 17), rendering the O2 molecule unreactive. If the reaction scheme outlined above is correct, then we should observe a quantum yield for photodestruction of ozone which is twice the quantum yield for production of O(lD) in the primary step. This is because O(3P) is assumed to undergo recombination (no net reaction) while O(lD) atoms each destroy a second O3 molecule in reaction 4. The primary O(lD) yield is reported to be 0.79 f 0.2 in the gas which is about half of our
(42) Davenport, J. E. Parameters for Ozone Photolysis as a Function of Temperature at 280-330 nm. Report No. FAA-EE-80-44, Contract No.
DOT-FAl8WA-4263.
(43) Arnold, I.; Comes, F. J.; Moortgat, G . K. Chem. Phys. 1977, 24, 21.
J. Phys. Chem. 1989, 93, 511-512 measured photodestruction yield of 1.5 f 0.2 in the solid state. This lends support to the proposed reaction mechanism. When ozone is trapped in excess ice, the measured photodestruction quantum yield is 0.4 f 0.2. Reaction of O(3P) with H20 to form hydrogen peroxide is exothermic, but spin forbidden. O('D) atoms can form HOOH or two OH radicals. In the solid state we expect that either mechanism would lead to the stabilization and/or formation of HOOH due to radical recombination in the reaction cavity.47 If we again make the hypothesis that O(lD)
+ H20
-
HOOH
(6)
O(3P) atoms undergo recombination with their partner O2molecules and only the channels producing O(lD) atoms are responsible for th: net destruction of ozone, we predict that the photodestruction quantum yield should be equal to the quantum yield for O(lD) formation in the primary step. Few direct measurements of this quantity are available. Taube19has reported the O('D) quantum yield of ozone in liquid water to be 0.21 at 3 13 nm, so our value of 0.4 f 0.2 at 308 nm is not unreasonable agreement. W e have observed IR absorption bands in the photolyzed 03/H20samples at 1405 and 2860 cm-' which are attributable to HOOH reaction product!8 These bands shift to 1075 and 2130 cm-' when the experiment is carried out with DzO, a result which (44) Brock, J. C.; Watson, R. T. Chem. Phys. 1980,46, 477. (45) Moortgat, G. K.; Kudszus, E. Geophys. Res. Lett. 1978, 5, 191. (46) Steinfeld, J. I.; Alder-Golden, S. M.; Gallagher, J. W. JILA Data Center Report No. 31, JILA, University of Colorado, 1987. (47) Cohen, M.D. Angew. Chem., Int. Ed. Engl. 1975, 14, 386. (48) Bain, 0.; Giguere, P. A. Can. J. Chem. 1954, 33, 527.
511
is consistent with formation of DOOD. It is worth noting that, in the gas phase, reaction of O(lD) with H20produces two OH radicals. These are chain reaction carriers which make the overall quantum yield for ozone destruction quite high. In the solid state, however, the HOOH molecule is stabilized as a final product. No chain reaction occurs, and the overall quantum yield for photodestruction of O3 is low. The experimental results suggest that the photolysis of O3in stratospheric polar ice crystals is not an efficient mechanism for ozone destruction because the solid itself inhibits the formation of OH chain reaction carriers. In addition, the antarctic atmosphere has a sharp wavelength cutoff at 310 nm, which is the threshold wavelength for producing O('D) atoms from ozone via spin-allowed reactions (Table 11). This may also limit the extent of ozone photochemistry since our results suggest that OPP) atoms undergo recombination to re-form 03.Consequently, the direct photodissociation of ozone in ice crystals is not likely to make a significant contribution to the overall observed depletion of atmospheric ozone. Experiments are now under way to elucidate the dependence of the photodestruction quantum yield on the relative concentrations of ozone and water in the solid samples. We are also investigating the wavelength dependence of the photochemistry to gain additional information about the electronic-state selectivity of reactions in the solid. Acknowledgment. We thank Professors D. J. Donaldson and Veronica Vaida for helpful discussions and Professor John Spikes for the loan of the UV absorption spectrometer. This research was supported by the U S . Air Force Astronautics Laboratory (Contract No. F04611-87-K-0023).
Scanning Tunneling Microscopic Imaging of Electropolymerized, Doped Polypyrrole. Visual Evldeuce of Semicrystalline and Helical Nascent Polymer Growth R. Yang, K. M. Dalsin, D. F. Evans, Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis. Minnesota 55455
L. Christensen, and W. A. Hendrickson* 3M Company, Corporate Research Laboratory, 3M Center, St. Paul, Minnesota 55144 (Received: November 8, 1988)
We have examined electropolymerized polypyrroles doped with p-toluenesulfonate (TOS),tetrafluoroborate (TFB), or trifluoroacetate (TFA), using scanning tunneling microscopy (STM). The initial stages of polymer growth involves microisland formation with concomitant polymer strands. In one case, the p-toluenesulfonate-dopedpolypyrrole, those strands have been shown to be helical.
Conducting polymers have been extensively investigated since the discovery in 1977 that doped polyacetylene was electrically conducting.' Polypyrrole is a particularly attractive material for commercial application because of its ease of formation* and relative stability in the doped, conductive form.3 Its physical properties depend on the dopant anion incorporated into the polymer4as well as the conditions of polymer formation. However, the amorphous and insoluble nature of the polymer makes structural characterization difficult. NMR, IR, and XPS studies on solid, thick films established that the pyrrole units are predominately bonded through a - l i n k a g e ~ . ~ .X-ray ~ diffraction analysis of 14-35-rm thick free-standing polypyrrole films shows *To whom correspondence should be addressed
that the polymers are macroscopically disordered and amorphous; however, the individual pyrrole units are coplanar with each other and are parallel to the original electrode surface? However, X-ray and TEM studies on thin films show that they grow in several (1) Shirakawa, H.; Louis, E. J.; MacDiarmid, A. G.; Chiang, C. K.; Heeger, A. J. J . Chem. Soc., Chem. Commun. 1977, 578. (2) Street, G. B. In Handbook of Conducting Polymers; Skotheim, T. A,, Marcel Dekker: New York, 1986; Vol. 1, Chapter 8. (3) Billingham, N. C.; Calvert, P. D.; Foot, P. J. S.; Mohammad, F. Polym. Degrad. Stab. 1987, 19, 323. (4) Reynolds, J. R. CHEMTECH 1988, 18(7), 440. (5) Street, G. B.; Clarke, T. C.; Krounbi, M.; Kanazawa, K.; Lee, V.; Pfluger, P.; Scott, J. C.; Weiser, G. Mol. Cryst. t i q . Cryst. 1982,83, 253-264. (6) Zeller, M. V.; Hahn, S. J. Surf.Interphase Anal. 1988, 327. (7) Mitchell, G. R.; Geri, A. J . Phys. D: Appl. Phys. 1987, 20, 1346.
0022-365418912093-0511%01.50/0 0 1989 American Chemical Society
